The present invention relates primarily to methods of improving the efficiency, and reducing operational and capital costs, of desalination systems. More particularly, to desalination systems that distill brackish or ocean water.
Fresh water is a scant 2.5% of the total global water supply and 69% of that is represented by permanent snow glaciers. The remaining 97.5% is salt water. Since 1940, the amount of fresh water used by humanity has roughly quadrupled as the world population doubled. Given the finite nature of the earth's fresh water resources, such a quadrupling of worldwide water use probably cannot occur again. In many of the regions where the world population is growing most rapidly, the needed water is not available. Desalination of seawater represents the best source of fresh water to satisfy needs.
Desalination Systems
Throughout the world today, all desalination facilities combined produce about 26.5 million cubic meters (approx. 7 billion gallons) of desalinated water per day. These facilities basically utilize only two technologies, membrane filter processes and thermal distillation processes. Of these processes reverse osmosis (membrane) and multi-stage flash distillation (thermal) make up and share about 80% of the world market.
Reverse Osmosis uses high pressure pumps to force fresh water through a semi-permeable membrane, leaving the salt behind. This process requires seawater pretreatment, an electrical power source, chemical post-treatment and annual membrane replacement.
Multi-Stage Flash (MSF) involves introducing heated sea water into multiple, reduced pressure chambers that cause a portion of the water to instantly flash (boil) into water vapor. The vapor is then condensed into distilled water. This process requires an energy source for heating the seawater as well as control functions.
Both technologies are energy intensive and both convert about 50% of the input sea water into fresh drinkable water, discharging the remaining brine solution back into the ocean, which results in an ever increasing environmental problem.
Other related technologies involve solar collectors (flat plate collectors, evacuated tube collectors, parabolic trough collectors) and low temperature flash desalination.
Flat plate collectors are used primarily for apartments and residential water heating applications. The efficiency of flat plate collectors depend mostly on the quality of insulation, the design of absorber plates, and heat transfer methods. Present day designs are limited to applications that require temperature less than 150° F. The collectors operate well even in hazy weather conditions.
Evacuated tube collectors are also used for apartments and residential water heating applications. Evacuated tube collectors concentrate the solar radiation on absorber tubes. The insulation quality of the evacuated tubes provides operating temperatures up to 200° F. The collectors are limited to clear weather conditions.
Parabolic trough collectors are more often used for applications that require high temperature for steam generation (electric power plants). Parabolic trough collectors use reflective surfaces to concentrate the solar radiation into liquid filled absorber tubes and provide operating temperatures above 600° F. The concentration of the energy onto the smaller absorber tubes provides methods for more efficient insulation and heat conservation. The collectors use automatic sun tracking to keep the collectors facing directly into the sun. The collectors are limited to clear weather conditions, require complex alignment, and are subject to high wind damage and maintenance cost.
Only a few large scale desalination plants throughout the world use solar energy as an energy source due to the large solar collector fields required and the associated high capital costs.
Low Temperature Flash Desalination: The past decade has seen a huge increase in research and development in desalination projects around the world utilizing improved technologies, resulting in efficiency and reduced capital costs. Numerous patents have been granted disclosing designs that improve efficiency. A large number of these patents involve the “flash desalination” of water at low, near ambient temperatures in an effort to reduce energy requirements. Although sea water can be evaporated at low temperatures by decreasing pressure (partial vacuum), the decreasing temperature results in an exponential decrease in the Vapor Saturation Density. Therefore, large quantities of vapor must be transferred to recover significant quantities of distilled liquid, which places much higher energy and costs requirements upon the vacuum system.